Network Working Group J. Snell
Internet-Draft June 30, 2010
Updates: 4287 (if approved)
Intended status: Informational
Expires: January 1, 2011
Atom Link Extensionsdraft-snell-atompub-link-extensions-07.txt
Abstract
This specification adds additional attributes to the Atom Syndication
Format link and content elements that may be used to express
additional metadata about linked resources.
Status of this Memo
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Internet-Draft Atom Link Extensions June 20101. Introduction
This specification adds additional attribute to the Atom Syndication
Format [RFC4287] link and content elements that may be used to
express additional metadata about linked resources.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119]
This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
to uniquely identify XML element names. It uses the following
namespace prefix for the indicated namespace URI;
"atom": "http://www.w3.org/2005/Atom"
3. Hash Attributes
Hash digest values are computed by the producers of Atom documents
and are representative of the state of linked resources at a given
point in time. The intent of providing hash values is to allow
consumers of the Atom document to later determine if linked resource
have been modified since the document was produced. There are,
however, many factors that determine whether a consumer of a document
will be capable of calculating a digest value identical to that
specified in a hash attribute. Accordingly, hash attribute values
MUST be considered to be strictly advisory. User agents SHOULD
notify users when matching hash digest values cannot be computed but
MUST NOT stop processing or signal an error.
3.1. Computing Hash Digests
When the resource referenced by atom:link or atom:content elements is
retrievable using HTTP, hash digest values are computed by first
performing an HTTP GET request on the URL specified by the @href or
@src attributes, extracting the returned entity-body, then following
the steps specified in Section 14.15 of [RFC2616].
3.2. The 'hash' attributes
The 'hash' Attribute specifies a whitespace-delimited list of hash
digest values calculated over the resource identified by the atom:
link/@href or atom:content/@src attributes. Each digest value is
represented as a token identifying the hash algorithm and the hex-
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Internet-Draft Atom Link Extensions June 2010
The presence and placement of the quotes in the entity tag value can
introduce some difficulty when inserting the value into the etag
attribute. Producers of Atom documents must either use single quotes
when specifying the value of the etag attribute, e.g.
etag='W/"xyzzy"' or use the &quot; entity reference to escape the
double quotes within the etag value, e.g. etag="W/&quot;xyzzy&quot;".
A strong entity tag would be encoded as either etag='"xyzzy"' or
etag="&quot;xyzzy&quot;".
5. The 'modified' attribute
The 'modified' Attribute specifies the date and time when the
resource identified by the atom:link or atom:content element was last
modified. The value MUST conform to the "date-time" production
defined by [RFC3339]. An uppercase "T" character MUST be used to
separate date and time, and an uppercase "Z" character MUST be
present in the absence of a numeric time zone offset. The 'modified'
attribute MAY appear as a child of the atom:link and atom:content
elements.
modified = attribute modified { xsd:dateTime }
An example last-modified attribute for an enclosed MP3 file:
<atom:link rel="enclosure"
href="http://example.org/media/myfile.mp3"
modified="2010-12-12T12:12:12Z" />
6. The 'accessed' attribute
The 'accessed' Attribute specifies the most recent date and time when
the resource identified by the atom:link or atom:content element was
accessed by the producer of the Atom document. The value MUST
conform to the "date-time" production defined by [RFC3339]. An
uppercase "T" character MUST be used to separate date and time, and
an uppercase "Z" character MUST be present in the absence of a
numeric time zone offset. The 'accessed' attribute MAY appear as a
child of the atom:link and atom:content elements.
accessed = attribute accessed { xsd:dateTime }
An example accessed attribute for an enclosed MP3 file:
<atom:link rel="enclosure"
href="http://example.org/media/myfile.mp3"
accessed="2010-12-12T12:12:12Z" />
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Internet-Draft Atom Link Extensions June 2010
The intent of the 'accessed' attribute is to allow the Atom document
producer to establish an explicit point-in-time at which additional
metadata about the linked resource was established. For instance, if
the 'accessed' attribute is used, a consuming user agent can assume
that any hash attribute values, entity tags and modified timestamps
were valid at the date and time specified by the 'accessed'
attributes value. If the 'accessed' attribute is not specified,
consumers SHOULD use the value of the atom:updated element to
determine the point-in-time at which the link metadata was considered
to be valid.
7. The 'media' attribute
The 'media' attribute identifies the optimum media for the resource
identified by the atom:link/@href attribute. The value MUST be a
valid media query as specified by the Media Queries Specification
[W3C.CR-css3-mediaqueries-20090915]. The media attribute MUST be
considered to be purely advisory and identifies for which type of
media the resource in question was designed. The 'media' attribute
MAY appear as a child of the atom:link element.
media = attribute media { media-query }
An example media attribute on an atom:link element:
<atom:link rel="alternate"
href="http://example.org/media/index.html"
media="handheld and (min-width: 20em)" />
Section 4.1.2 of the Atom specification [RFC4287] limits the ability
for an atom:entry to contain multiple atom:link elements with a rel
attribute value of "alternate". Specifically, the specification
states that an atom:entry "MUST NOT contain more than one atom:link
element with a rel attribute value of "alternate" that has the same
combination of type and hreflang attribute values." This restriction
limits the ability of an Atom document publisher to provide multiple
media-targeted alternate links of the same content type. Each
alternate link MUST specify a different media type or language.
Other types of links do not share the same limitation.
8. Link extension values and the 'atom:updated' element
Changes in the values of the 'hash', 'etag', 'modified' and
'accessed' attributes MUST be considered by Atom document producers
to be "significant" changes as discussed in Section 4.2.15 of
[RFC4287] and MUST result in a change to the value of the 'atom:
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Internet-Draft Atom Link Extensions June 2010
updated' element.
9. Security Considerations
The 'hash', 'etag' and 'modified' attributes are intended to allow an
Atom publisher the means of describing the state of a linked resource
at a point-in-time -- usually at the moment specified by the
'accessed' attribute or at the moment specified by the atom:updated
element. An Atom consumer that is aware of these attributes can use
their values as an integrity check to determine if the linked
resource has been modified since the attribute values were
established by the publisher.
The 'hash' attribute is intended for use when the publisher of an
Atom document requires the ability to link to a specific version of a
resource that is expected to remain stable and unchanged for a useful
period of time. If publishers fall into the habit of regularly
including hash digests for resources whose states change frequently,
there is a danger that consumers of feeds containing large numbers of
invalid digests will simply begin to ignore them and completely
undermine the utility of the attribute.
10. IANA Considerations
No IANA actions are required by this document.
11. Normative References
[RFC1864] Myers, J. and M. Rose, "The Content-MD5 Header Field",
RFC 1864, October 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, December 2005.
[W3C.CR-css3-mediaqueries-20090915]
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